1. Field of the Invention
The present invention relates to noninvasive systems for monitoring blood glucose and other blood constituent concentrations.
2. Description of the Related Art
In the past, many systems have been developed for monitoring blood characteristics. For example, devices have been developed which are capable of determining such blood characteristics as blood oxygenation, glucose concentration, and other blood characteristics. However, significant difficulties have been encountered when attempting to determine blood glucose concentration accurately using noninvasive blood monitoring systems.
The difficulty in determining blood glucose concentration accurately may be attributed to several causes. First, blood glucose is typically found in very low concentrations within the bloodstream (e.g., on the order of 100 to 1,000 times lower than hemoglobin) so that such low concentrations are difficult to detect noninvasively, and require a very high signal-to-noise ratio. Second, there has been a lack of recognition of the kinds of noise and the proper method to use when removing this noise. For example, noise can be classified as deterministic (definable) or stochastic (random) where either of these kinds of noise could be linear (added) or modulated (multiplied). Knowledge of the distinction between the various kinds of noise is essential for purposes of using the proper method of removing noise. Additionally, the optical characteristics of glucose are very similar to those of water which is found in a very high concentration within the blood. Thus, where optical monitoring systems are used, the optical characteristics of water tend to obscure the characteristics of optical signals due to low glucose concentration within the bloodstream. Furthermore, since each individual has unique blood properties, each measurement typically requires calibration for the particular individual.
In an attempt to accurately measure blood glucose levels within the bloodstream, several methods have been used. For example, one method involves drawing blood from the patient and separating the glucose from the other constituents within the blood. Although highly accurate, this method requires drawing the patient's blood, which is less desirable than noninvasive techniques, especially for patients such as small children or anemic patients. Furthermore, when blood glucose monitoring is used to control the blood glucose level, blood must be drawn three to six times per day, which may be both physically and psychologically traumatic for a patient. Other methods contemplate determining blood glucose concentration by means of urinalysis or some other method which involves pumping or diffusing blood fluid from the body through vessel walls. However, such an analysis tends to be less accurate than a direct measurement of glucose within the blood, since the urine, or other blood fluid, has passed through the kidneys. This problem is especially pronounced in diabetics. Furthermore, acquiring urine samples is often inconvenient.
Another proposed method of measuring blood glucose concentration is by means of optical spectroscopic measurement. In such devices, light of multiple wavelengths may be used to illuminate a relatively thin portion of tissue, such as a fingertip or an earlobe, so that a spectrum analysis can be performed to determine the properties of the blood flowing within the illuminated tissue. Although such a method is highly desirable due to its noninvasive character and its convenience to the patient, problems are associated with such methods due to the difficulty in isolating each of the elements within the tissue by means of spectroscopic analysis. The difficulty in determining blood glucose concentration is further exacerbated due to the low concentration of glucose within blood, and the fact that glucose in blood has very similar optical characteristics to water. Thus, it is very difficult to distinguish the spectral characteristics of glucose where a high amount of water is also found, such as in human blood.
As is well known in the art, different molecules, typically referred to as constituents, contained within the medium have different optical characteristics so that they are more or less absorbent at different wavelengths of light. Thus, by analyzing the characteristics of the fleshy medium at different wavelengths, an indication of the composition of the fleshy medium may be determined.
Spectroscopic analysis is based in part upon the Beer-Lambert law of optical characteristics for different elements. Briefly, Beer-Lambert's law states that the optical intensity of light through any medium comprising a single substance is proportional to the exponent of the path lengths through the medium times the concentration of the substance within the medium. That is, EQU I=I.sub.o e.sup.-(pl.multidot.c) (1)
where pl represents the path length through the medium and c represents the concentration of the substance within the medium. For optical media which have several constituent substances, the optical intensity of the light received from the illuminated medium will be proportional to the exponent of the path length through the medium times the concentration of the first substance times an optical absorption coefficient associated with the first substance, plus the path length times the concentration of the second substance times the optical absorption coefficient associated with the second substance, etc. That is, EQU I=I.sub.o e.sup.-(pl.multidot.c.sbsp.1.sup..multidot..epsilon..sbsp.1.sup.+pl.multid ot.c.sbsp.2.sup..multidot..epsilon..sbsp.2.sup.+etc.) (2)
where represents the optical absorption coefficient.